While the trip was shrouded in secrecy for security reasons, an airplane aficionado spotted the distinctive blue and white Boeing jet flying over England, and posted pictures of it on his Flickr account Wednesday morning.

When the president is on board, the VC-25A plane — a military version of the 747 — uses the call sign Air Force One. The plane spotted over England, however, was using the call sign RCH358.

Online detectives figured out the plane's disguised call numbers, and tracked its movements — even sharing them on Twitter.

Is Trump on a surprise visit to Afghanistan? Turkey? Flight trackers show that one of two planes used for Air Force One (92-9000/VC-25) using what appears to be a fake HEXCODE of AE47C4, departed Andrews Air Force base at midnight. Transponders changed or disabled near Romania. pic.twitter.com/JtVzEPUoLv

WikiLeaks then got in on the act, tweeting out a map of the route the plane was taking. "The Boeing 747 usually used by Trump for Air Force One is currently moving towards Turkey," one of the tweets said.

That led to speculation online that Trump was going to visit troops in Afghanistan, or to pay his respects to Turkish President Recep Tayyip Erdogan, who reportedly recently told the president that Turkey would take responsibility for finishing off the Islamic State if the U.S. pulled out of Syria.

WikiLeaks then tweeted that the plane's transponders — the device used to to track plane flights — were either changed or disabled near Romania.

Paul Reickhoff, an Iraq War veteran and founder and CEO of Iraq and Afghanistan Veterans of America, questioned the White House's operational security measures on Twitter, saying it's "a bit troubling that so many folks seem to already know about this if it hasn't happened already."

Sources telling me Trump’s on his way to visit troops—possibly in Iraq. Better late than never. But also a bit troubling that so many folks seem to already know about this if it hasn’t happened already. #OpSec anyone?

The online tracking appeared to thwart, to an unusual degree, the security protocols that often have kept the plane's movements obscured on visits to war zones in the past.

The plane's shades were drawn and lights were out for much of the 11-hour trip for security reasons, according to a Reuters reporter who was on the flight. Trump, meanwhile, stayed off Twitter for the duration of the flight — which to many on social media was also a tell that something was up, given his prolific tweeting in recent days.

Asked if he'd had any concerns about making the trip, Trump said, "Absolutely."

"I had concerns for the institution of the presidency, because, not for myself personally, I had concerns for the first lady, I will tell you," he said.

"But if you would have seen what we had to go through, with the darkened plane with all windows closed, with no lights on whatsoever, anywhere, pitch black, I’ve never seen that, been on many airplanes, all types and shapes and sizes, I’ve never seen anything like it."